I agree with what Goon said. Lead .44's might be a it heavy for a wrist rocket, but either smaller-caliber or alternative materials might work a bit better.
Option 2 is to get "magnum" slingshot rubber. These are thicker than the stock rigs, and hit harder, but require more elbow grease.
It is worth noting that commercial W.R. ammo comes in two grades: .375 steel, and .250 steel, niether of which appeal to me. Not big enough, weight-wise. .375 lead might solve that, but I was going to go to a craft store and see if I could find cheap bulk marbles for use in flower arranging. Good size, nice weight, consistent flight, and cheap by the hundreds. I like marble-sized creek pebbles better than dinky steels, but consistency is an issue. Cost, also. $2.99/50 is a bit much. .250's were the same for a 250-count, but they're WAY too light.
Now $2.99/250 1/2"-5/8" marbles would suit me fine. I could afford to practice, and not wince about losing a few. Then them 'coons had best worry, as I need a retro hat. If'n I don't block the dogdoor at night, the arrogant bastards come in the HOUSE, and I can't lie in wait, as ricochets off skulls would take out the sliding glass doors.
Fun-but-useless: 1/4" square or so FLAT chips of stone, bounced off the ground at a shallow angle for movie-style ricochet sound effects.
Interesting alternative ammo: Rings of tape, diameter of your choice, lead birdshot, and glue or wax. Hits hard, but no ricochet issues in urban areas to damage neighbor's cars/windows. Haven't tried this, just thought it up right now.
Alternative #2: Tape a stick across the fork for upright, or along the front of the grip for sideways use, and run arrows.
Dangerous plans: If I can get a source for heavy rubber tubing, I plan on making a rifle-style setup with at LEAST a 4-foot draw and a trigger release with sights. Run THAT beast with lead bullets and I could run afoul of Kaliforny firearms law if I ran into the wrong cop. Maybe I could use Bungees. I wonder what kind of FPS I could get...
edit: 'pebbles', not 'pepples'